Unit cell of display panel including integrated TFT photodetector

ABSTRACT

A unit pixel arranged along with a display pixel in each pixel of a display panel is provided. The unit pixel may include a thin-film transistor (TFT) photodetector including an active layer formed of amorphous silicon or polycrystalline silicon on an amorphous transparent substrate, and at least one transistor electrically coupled to the TFT photodetector and configured to generate a voltage output from photocurrent generated from the active layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Application No.62/889,560 filed on Aug. 20, 2019, which is incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a unit pixel of a display panelincluding an integrated thin film transistor (TFT) photodetector, andmore particularly, to a unit pixel stacked with a display pixel orarranged side by side with the display pixel, in each pixel of a displaypanel.

BACKGROUND

Technologies such as liquid crystals, organic light emitting diode(OLED) cells, touch screens, backlights, and thin film transistors(TFTs) on glass are integrated on a display panel. Particularly, thetrend of recent mobile devices is toward a display panel which tends tobe as large as or larger than an overall device size, and a displayitself is becoming more flexible.

However, the current display system performs only a one-way function ofoutputting an image or the like to the outside, without a function ofefficiently, directly acquiring an input signal. At present, the displaysystem executes only a touch screen function, while a separate imagesensor performs a process such as image sensing.

Particularly in a mobile device or a laptop computer to which abiometric recognition and authentication system such as fingerprint orface recognition and authentication is essential, there aretechnological limitations in acquiring a signal from an image sensorconfined to any specific position on a display. Although it is mostdesirable to incorporate an input signal device into the display system,an image sensor cannot be implemented on a display panel with thecurrent technology, thus making it impossible to integrate the displaypanel with the image sensor in real implementation.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

SUMMARY

An aspect of the present disclosure is to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentdisclosure is to provide a high-sensitivity image sensor on a glasssubstrate or a flexible substrate such as a polyimide film, which isused as a display anel, by a thin-film transistor (TFT) fabricationtechnology.

Another aspect of the present disclosure is to enable a display moduleto function as an image sensor without the need for a separate imagesensor on the display panel.

Another aspect of the present disclosure is to perform an image sensingprocess without the need for separately providing a light emitting partfor an image sensor, by using a light emitting device or backlight unit(BLU) of a display as a light source for the image sensor.

Another aspect of the present disclosure is to realize a transparentdisplay panel capable of displaying and image sensing by stacking ascreen panel of a display and an image sensing panel or arranging thescreen panel and the image sensing panel on the same panel.

Another aspect of the present disclosure is to fabricate a switching TFTfor display and a driving TFT for image sensing in a single process byarranging a screen panel and an image sensing panel on the same panel.

Another aspect of the present disclosure is to use a light source for adisplay also as a light source for an image sensor.

Another aspect of the present disclosure is to use both of a BLU of aliquid crystal display (LCD) and a light emitting source of an organiclight emitting diode (OLED).

In accordance with an aspect of the present disclosure, a unit pixelarranged along with a display pixel in each pixel of a display panel mayinclude a TFT photodetector including an active layer formed ofamorphous silicon or polycrystalline silicon on an amorphous transparentsubstrate, and at least one transistor electrically coupled to the TFTphotodetector and configured to generate a voltage output fromphotocurrent generated from the active layer.

The TFT photodetector may be configured in a structure in which whenlight is incident, electrons are introduced into an N-type gate bytunneling from a P-type active layer to an insulating oxide film, amongcharges of two PN areas excited with the insulating oxide film inbetween, the electron migration changes a threshold voltage of a currentchannel between a source and a drain in correspondence with a change inthe total amount of charge in the gate, the photocurrent proportional tothe intensity of the received light flows in the active layer, and thevoltage output is generated form the flowing photocurrent.

The active layer may contain a material with a conductive propertycontrollable by tunneling or an electric field.

The active layer may contain at least one of amorphous silicon orpolycrystalline silicon.

The at least one transistor may convert the photocurrent into thevoltage output based on parasitic capacitance between the at least onetransistor, caused by the photocurrent generated from the active layer.

The at least one transistor may include a select transistor, and whenthe select transistor is turned on, the parasitic capacitance may begenerated.

The at least one transistor may convert photocurrent into the voltageoutput, the photocurrent being generated by directly charging acapacitor coupled to a transistor corresponding to a source followerwith the photocurrent generated from the active layer.

The at least one transistor may include a select transistor. When theselect transistor is turned on, a capacitor of an IVC circuit may becharged, among capacitors coupled to the transistor corresponding to thesource follower, and the charged photocurrent may be converted into avoltage which is an output of the IVC circuit.

A pixel structure may be determined based on a thickness of the activelayer.

The unit pixel may further include a transistor coupled to the activelayer and configured to control residual charges in a neutral area, whenthe thickness of the active layer is equal to or larger than a referencevalue.

The reference value may be 100 nm.

The TFT photodetector may include an amorphous transparent substrateused as a display panel, a source formed of amorphous silicon orpolycrystalline silicon on the transparent substrate, a drain formed ofamorphous silicon or polycrystalline silicon, opposite to the source onthe transparent substrate, an active layer formed between the source andthe drain and having a current channel formed between the source and thedrain, an insulating oxide film formed on the source, the drain, and theactive layer, and a light receiving part formed on the insulating oxidefilm, isolated from the active layer by the insulating oxide film, andconfigured to absorb light.

In the TFT photodetector, when light is incident on the light receivingpart, electrons may migrate by tunneling through the insulating oxidefilm between the light receiving part and the active layer which havebeen excited with the insulating oxide film in between, the amount ofcharge in the light receiving part may be changed by the migration ofthe electrons, a threshold voltage of the current channel may be changeddue to the change of the amount of charge, and photocurrent may flowthrough the current channel due to the change of the threshold voltage.

The TFT photodetector may use light generated from the display panel asa light source for a sensor.

In another aspect of the present disclosure, a unit pixel of a displaypanel may include a display sub-panel including a light emitting device,and an image sensor sub-panel stacked with the display sub-panel, andconfigured to generate a voltage output from photocurrent generated froman active layer in response to sensed light. The image sensor sub-panelmay include a TFT photodetector including an active layer formed ofamorphous silicon or polycrystalline silicon on an amorphous transparentsubstrate, and at least one transistor electrically coupled to a sourceside of the TFT photodetector and configured to generate a voltageoutput from photocurrent generated from the active layer.

The image sensor sub-panel may be configured in a structure in whichwhen light is incident, electrons are introduced into an N-type gate bytunneling from a P-type active layer to an insulating oxide film, amongcharges of two PN areas excited with the insulating oxide film inbetween, the electron migration changes a threshold voltage of a currentchannel between a source and a drain in correspondence with a change inthe total amount of charge in the gate, the photocurrent proportional tothe intensity of the received light flows in the active layer, and thevoltage output is generated form the flowing photocurrent.

In another aspect of the present disclosure, a unit pixel of a displaypanel may include a display sub-panel including a light emitting device,and an image sensor sub-panel formed near to the display sub-panel onthe same layer, and configured to generate a voltage output fromphotocurrent generated from an active layer in response to sensed light.The image sensor sub-panel may include a TFT photodetector including anactive layer formed of amorphous silicon or polycrystalline silicon onan amorphous transparent substrate, and at least one transistorelectrically coupled to a source side of the TFT photodetector andconfigured to generate a voltage output from photocurrent generated fromthe active layer.

The unit pixel may further include a driving switch. The driving switchmay control driving of the display sub-panel or control the image sensorsub-panel.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of certainexemplary embodiments of the present disclosure will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a display module with thinfilm transistor (TFT) photodetectors implemented thereon, which is usedas an image sensor, in an electronic device according to an embodimentof the present disclosure;

FIG. 2 is a diagram illustrating an exemplary TFT photodetectorimplemented on a pixel basis according to an embodiment of the presentdisclosure;

FIG. 3 is a sectional view illustrating exemplary implementations of aTFT photodetector on a pixel basis on a display according to anembodiment of the present disclosure;

FIG. 4 is a sectional view illustrating a TFT photodetector according toan embodiment of the present disclosure;

FIG. 5 is sectional views illustrating a process of fabricating a TFTphotodetector according to an embodiment of the present disclosure;

FIG. 6 is an energy band diagram illustrating a photo-electricconversion mechanism of a TFT photodetector according to an embodimentof the present disclosure;

FIG. 7 is an energy band diagram illustrating a tunneling mechanism of aTFT photodetector according to an embodiment of the present disclosure;

FIG. 8 is a diagram illustrating a photo-electric conversion mechanismcaused by a plurality of localized states in a TFT photodetector formedof amorphous silicon (a-Si) or polycrystalline silicon (poly-Si);

FIGS. 9 and 10 are diagrams illustrating unit pixels, when the thicknessof an active layer is equal to or less than 100 nm;

FIGS. 11 and 12 are diagrams illustrating unit pixels, when thethickness of an active layer is equal to or larger than 100 nm;

FIG. 13 is a diagram illustrating an active pixel system (APS)-type cellarray in which unit pixels are arranged to provide a voltage output to adriving circuit; and

FIG. 14 is a diagram illustrating a passive pixel system (PPS)-type cellarray in which unit pixels are arranged to provide a current output to adriving circuit

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION

The present disclosure will be described in detail with reference to theattached drawings. Lest it should obscure the subject matter of thepresent disclosure, a known technology will not be described in detail.An ordinal number (e.g., first, second, and so on) used in thedescription of the present disclosure is used simply to distinguish onecomponent from another component.

When it is said that one component is “coupled to or with” or “connectedto” another component, it is to be understood that the one component maybe coupled to or connected to the other component directly or with athird party in between.

FIG. 1 is a schematic diagram illustrating a display module with thinfilm transistor (TFT) photodetectors implemented thereon, which is usedas an image sensor, in an electronic device according to an embodimentof the present disclosure.

A TFT photodetector 100 according to the present disclosure is formed ona display panel 200 of an electronic device 10. The electronic device 10may be any of devices equipped with a display, such as a smartphone, alaptop computer, a monitor, or a TV.

Specifically, TFT photodetectors 100 may be formed across the whole orpart of the display panel 200, and a TFT photodetector 100 may be formedin each individual pixel, thus operating as a part of the pixel. WhenTFT photodetectors 100 are formed across the whole display panel 200,the number of the TFT photodetectors 100 may be equal to the number ofpixels corresponding to the resolution of the display panel 200. Thedisplay panel 200 may be any of a light receiving display requiring abacklight unit (BCU), such as a liquid crystal display (LCD) or a lightemitting display which emits light on its own, such as a light emittingdiode (LED) (e.g., organic LED (OLED) or active matrix OLED (AMOLED))display or a plasma display panel (PDP).

The display panel 200 displays a video or an image or operates as animage sensor, according to an operation of the electronic device 10.When the display panel 200 operates as an image sensor, an optical imageof an external object may be acquired by means of a plurality of TFTphotodetectors 100 implemented on the display panel 200. A light sourcerequired for image sensing may be an external light source such asnatural light or an external lighting, or an internal light source suchas a BCU or OLED elements of the display panel 200.

As such, formation of TFT photodetectors 100 of the present disclosureon the display panel 200 advantageously enables use of the display panel200 as an image sensor without the need for providing a separate imagesensor in the electronic device 10. Further, because the display panel200 is used as an image sensor, a light source for display may also beused as a light source for image sensing without the need for adding alight source for image sensing in the electronic device 10. Therefore,the effects of device simplification and reduced fabrication cost may beexpected.

Further, because the pixels of the image sensor are formed in the samesize as the pixels of the display, as many image sensor pixels as thenumber of pixels corresponding to the resolution of the display may bearranged in the electronic device 10. In this case, the whole displaymay serve as an image sensor. The electronic device 10 may acquire animage of an external object by controlling image sensor pixels in thewhole or part of the display. Hereinbelow, an image sensor pixel isinterchangeably used with a unit pixel of the image sensor. Obviously,the TFT photodetector 100 of the present disclosure is formed in a unitpixel of the image sensor. Further, a unit pixel of the display panel isinterchangeably used with a display pixel. The unit pixel will bedescribed in detail with reference to FIG. 9 .

Further, the electronic device 10 may acquire biometric informationabout an external object, such as information about the fingerprint of afinger, a finger vein, a face, or an iris by the display panel 200 withTFT photodetectors 100 formed thereon. For example, a user may touch hisor her finger on any area of the display or place the finger within apredetermined distance from the area of display in the electronic device10, so that a fingerprint image may be acquired from a plurality ofimage sensor pixels formed in the area of the display. Throughout thespecification, the display panel 200 may be referred to as a display ora screen panel of a display.

Now, a description will be given of formation of TFT photodetectors on adisplay panel.

FIG. 2 is a diagram illustrating exemplary formation of a TFTphotodetector in each pixel of a display according to an embodiment ofthe present disclosure.

Although the TFT photodetector 100 of the present disclosure operates ina similar principle to that of a photo assisted tunneling-photodetector(PAT-PD) disclosed in U.S. patent application Ser. No. 15/885,757, theTFT photodetector 100 and the PAT-PD are different in that the PAT-PD isformed on a single crystalline silicon substrate, and an active layer, asource, a drain, and a gate serving as a light receiving part are formedof single crystalline silicon, whereas the TFT photodetector 100 isformed on the display panel 200 which is a glass substrate or atransparent flexible substrate using a transparent film formed of, forexample, polyimide (PI), polyethylene terephthalate (PET), polypropylene(PP), polycarbonate (PC), polymethylmethacrylate (PMMA),polyethylenenaphthalate (PEN), polyetheretherketone (PEEK),polyethersulfone (PES), or polyarylite, and an active layer, a source, adrain, and a light receiving part are formed of amorphous silicon (a-Si)or polycrystalline (poly-Si) silicon. Glass or a PI film is amorphous,which makes it impossible to stack single crystalline silicon thereon.Therefore, when TFT photodetectors are formed on a glass substrate or aflexible substrate, the TFT photodetectors should be implemented in anamorphous or polycrystalline fashion. Under circumstances, the amorphoussilicon or the polycrystalline silicon may be replaced with a materialwith a conductive property controllable by an electric field ortunneling. Throughout the specification, the term “PAT-PD” or “TFTPAT-PD” is interchangeably used with “TFT photodetector”.

Preferably, display pixels and image sensor pixels are matched to eachother in a one-to-one correspondence. FIG. 2 illustrates an exemplarypixel structure of the display panel 200 with TFT photodetectors 100formed thereon. A unit pixel 300 of the display panel 200 includes alight emitting area 310 for display, a switching TFT 320, and a TFTphotodetector 100 for image sensing. The display panel 200 may bedesigned such that each unit pixel of the display panel and each unitpixel of the image sensor occupy areas of similar sizes and thus thedisplay pixels and the image sensor pixels may be matched in aone-to-one correspondence per position. In this case, as the TFTphotodetector 100 may operate, using the light emitting area 310 of thedisplay pixel as a light source, a signal may be processed by matchingthe light emitting area 310 to the TFT photodetector 100, and data maybe processed by matching data included in the light source to datacollected by the TFT photodetector 100.

Although it is preferable to form the TFT photodetector 100 without anyoverlap with the light emitting area 310, the TFT photodetector 100 maybe formed overlapping with the light emitting area 310 over apredetermined area because the TFT photodetector 100 occupies a smallarea relative to the light emitting area 310. However, to maximizephotoelectric conversion, the introduction of unnecessary light isblocked by shielding an area except for the light receiving part of theTFT photodetector 100 with a metal or the like. The resulting shieldingof a part of the light emitting area 310 with the light shielding areaexcept for the light receiving part of the TFT photodetector 100 maydecrease the light emission efficiency of the display.

In some cases, the display pixels and the image sensor pixels may beconfigured in different sizes. For example, when the unit pixels of theimage sensor are designed such that one display pixel area correspondsto n image sensor pixels, n TFT photodetectors 100 share the lightemitting area of one display pixel as a light source, making itdifficult to control the TFT photodetectors 100 individually by lightsource control. However, the light source control may be simplified,which in turn simplifies an image sensing process. On the contrary, theunit pixels of the image sensor may be designed such that the area ofone unit pixel of the image sensor corresponds to m display pixels. Inthis case, although fewer image sensor pixels than the number of pixelscorresponding to the resolution of the display may be arranged, one TFTphotodetector 100 uses the light emitting areas of m display pixels aslight sources, and thus fine light source control required for imagesensing and data processing may become difficult.

The light emitting area 310 may be formed in a different structureaccording to the type of a used display For example, when the displaypanel 200 of the electronic device 10 is a light emitting display suchas an organic light emitting diode (OLED) display, the light emittingarea 310 may be a light emitting pixel with red, green, blue (RGB)sub-pixels arranged therein. When the display panel 200 of theelectronic device 10 is a light receiving display such as a liquidcrystal display (LCD), RGB sub-filters may be arranged in the lightemitting area 310. Obviously, the TFT photodetector 100 may use anexternal light source such as natural light as a light source for imagesensing, instead of the light emitting area 310.

With reference made to FIG. 2 again, a plurality of unit pixels 300 arearranged in a lattice structure. Each unit pixel 300 may be formed byvertically stacking or arranging side by side a display sub-panel formedon a glass substrate or a transparent flexible substrate and an imagesensor sub-panel formed on a glass substrate or a transparent flexiblesubstrate. In this regard, FIG. 3 illustrates a cross section of a unitpixel 300 of the display.

Referring to FIG. 3 , the unit pixel 300 of the display panel includes adisplay sub-panel 330 and an image sensor sub-panel 340. The displaysub-panel 330 may include a light emitting area 310 for display and itsswitching TFT 320, and the image sensor sub-panel 340 may include a TFTphotodetector 100 for image sensing and a detector driving TFT 344 fordriving the TFT photodetector 100.

For example, the detector driving TFT 344 may include at least onetransistor which is electrically coupled to a source side of the TFTphotodetector 100, and generates a voltage output from photocurrentgenerated in the active layer of the TFT photodetector 100.

The display sub-panel 330 or the image sensor sub-panel 340 is formed ona transparent glass substrate or a transparent flexible substrate suchas a PI film (hereinafter, a glass substrate and a transparent substrateare interchangeably used with each other). The transparent display panel200 may be formed by vertically stacking and attaching the two panels asillustrated in FIG. 3(a) or arranging the two panels side by side on thesame glass substrate 334 as illustrated in FIG. 3(b).

Particularly, the image sensor sub-panel 340 may be stacked with thedisplay sub-panel 330 in the structure of FIG. 3(a). Further, inresponse to light sensed by the image sensor sub-panel 340, a voltageoutput may be generated from photocurrent generated from the activelayer.

When light is incident on the image sensor sub-panel 340, electrons areintroduced into an N-type gate by tunneling from a P-type active layerto an oxide film, among charges of two PN areas excited with the oxidelayer in between. The electron migration changes the threshold voltageof a current channel between a source and a drain in correspondence witha change in the total amount of charge in a gate, and thus photocurrentcorresponding to the intensity of the incident light flows in the activelayer. Further, the image sensor sub-panel 340 may generate a voltageoutput from the flowing photocurrent.

Alternatively, the light emitting area 310 and the switching TFT 320 ofan OLED device for display, and the TFT photodetector 100 for imagesensing and the detector driving TFT 344 may be arranged together on thesame glass substrate 332 or 342, as illustrated in FIG. 3(b). In thiscase, a driving switch 322 may be formed by incorporating a switchingTFT for controlling the light emitting area 310 with a switching TFT forcontrolling the TFT photodetector 100, or the switching TFTs may beformed separately.

Throughout the specification, the display sub-panel 330 and the imagesensor sub-panel 340 may also be referred to as a display pixel and animage sensor pixel, respectively.

As described before, the image sensor pixel 340 of a similar size tothat of the display pixel 330 senses light and acquires an image bysignal processing and detector driving, and includes the TFTphotodetector 100 and the detector driving TFT 344 for driving the TFTphotodetector 100. The switching TFT 320 for an output to be used fordisplay, and the detector driving TFT 344 for driving the TFTphotodetector 100 formed on an image sensor pixel basis may beintegrated or configured separately. In this manner, the TFTphotodetector 100 of the present disclosure is formed on a pixel basis.

Because the TFT photodetector 100 should be formed on an amorphoussubstrate such as a glass substrate or a PI film, not a singlecrystalline silicon substrate, the TFT photodetector 100 should beimplemented in a different manner from an existing photodetector usingsingle crystalline silicon. Now, a description will be given of adetailed structure, operation mechanism, fabrication method of a TFTphotodetector according to the present disclosure.

FIG. 4 is a sectional view illustrating a TFT photodetector according toan embodiment of the present disclosure.

Referring to FIG. 4 , the TFT photodetector 100 of the presentdisclosure is formed on the transparent substrate 342 such as anamorphous glass substrate or a flexible substrate, and includes a gate150 formed of a-Si or poly-Si on the transparent substrate 342, aninsulating oxide film 140 capable of controlling tunneling of opticallyexcited charges, a drain 110, a source 120, and an active layer 130 inwhich a current channel is formed between the source 120 and the drain110. While the drain 110, the source 120, the active layer 130, and thegate 150 are formed of a-Si or poly-Si, they may be formed of any othermaterial as far as the material has a conductive property controllableby tunneling or an electric field.

The gate 150 is formed of N-type poly-Si or a-SI by implanting an N-typeimpurity and operates as a light receiving part that absorbs incidentlight. The active layer 130 is formed of P-type poly-Si or a-Si, withthe insulating oxide film 140 between the active layer 130 and the gate150. The active layer 130 forms a current channel according to opticalexcitation between the drain 110 and the source 120 which are P+-typediffusion layers.

An area on which light is incident is confined to the gate 150 servingas the light receiving part and the active layer 130 between which andthe gate 150 the insulating oxide film 140 is interposed. For thispurpose, a metal protection layer 160 may be formed on a boundarysurface of the transparent substrate 342, except for the area betweenthe transparent substrate 342 and the active layer 130, to shieldunnecessary light introduced into the TFT photodetector 100. A metalshielding layer 170 may be formed in the remaining area except for thegate 150 in an upper part of the TFT photodetector 100. The shieldinglayer 170 may be formed by a silicide and metal process. The TFTphotodetector 100 limits an area on which light is incident to the gate150 serving as the light receiving part by means of the shielding layer170, thereby maximizing the photoelectric change of the gate 150.Hereinbelow, the gate 150 and the light receiving part areinterchangeably used throughout the specification.

In a state where no light is introduced, the TFT photodetector 100controls biases of the gate 150, the drain 110, the source 120, and theactive layer 130 to maintain a stable equilibrium state in whichelectrons are trapped. For this purpose, the metal protection layer 160is provided on the boundary surface between the overlying shieldinglayer 170 and the transparent substrate 342 to shield unintendedunnecessary light through the transparent substrate 342 of, for example,glass. Specifically, the active layer 130 between the source 120 and thedrain 110 is bias-controlled to have a threshold voltage at which thepotential state of a silicon surface on which a current channel may beformed in an initial fabrication process is shortly before asub-threshold state. In this state, when light is not incident on thegate 150 as the light receiving part, photocurrent does not flow in thecurrent channel.

When light is incident on the light receiving part, electrons areintroduced into the N-type gate 150 by tunneling from the P-type activelayer 130 to the insulating oxide film 140, among charges of the two PNareas excited with the insulating oxide layer 140 in between, theelectron migration changes the threshold voltage of the current channelbetween the source 120 and the drain 110 in correspondence with a changein the total amount of charge in the gate 150, the threshold voltagemodulation effect caused by the change in the amount of charge in thelight receiving part causes a change in the conductance of the currentchannel, and thus photocurrent corresponding to the changed conductanceflows.

Since the gate 150 is doped with holes, the electrons passed through theinsulating oxide film 140 by tunneling are combined with holes in anarea of the gate 150 near to the insulating oxide film 140, therebygenerating a depletion layer at the top end of the insulating oxide film140. Therefore, the threshold voltage drops due to a change in thecharge of the active layer 130 near to the insulating oxide film 140,thereby exciting the current channel between the source 120 and thedrain 110.

In other words, current that flows in the current channel excitedbetween the source 120 and the drain 110 by light reception at the lightreceiving part is not a direct flow of charges of electron-hole pairs(EHPs) caused by the light reception but an indirect current flow in thecurrent channel excited by tunneling of directly generated charges.Therefore, a very high-efficiency light detection capability may beachieved.

FIG. 5 is a sectional view illustrating a process of fabricating a TFTphotodetector according to an embodiment of the present disclosure.

In FIG. 5(a), the P-type poly-Si or a-Si diffusion layer 130 to be usedas an active layer is formed on the glass substrate 342 or a flexiblesubstrate of, for example, a PI film, and two P+-type diffusion layers111 and 121 are formed of a-SI or poly-Si at both sides of the diffusionlayer 130.

The diffusion layers 130, 111, and 121 may be formed of a-SI or poly-Si.To increase mobility, the diffusion layers 130, 111, and 121 may beformed by depositing a-SI and then crystalizing the deposited a-Si intopoly crystals by thermal treatment such as laser annealing, or directlydepositing poly-Si on a transparent substrate.

Subsequently, a thin SiO₂ or SiNx insulating oxide layer 141 is formedon the diffusion layers 130, 111, and 121. The insulating oxide layer141 may be formed by sputtering or plasma enhanced chemical vapordeposition (PECVD).

Subsequently, an N-type diffusion layer 151 is formed of poly-SI or a-Sion the insulating oxide film 141 in the same manner.

Referring to FIG. 5(b), the gate 150 is then formed for use as a lightreceiving part by photo-patterning the generated diffusion layer 151.Referring to FIG. 5(c), only the generated insulating oxide layer 141 isetched away, remaining only a necessary part by a photoresist (PR)patterning process. Partial insulating oxide films 142 and 143 areremoved together on areas of the diffusion layer 121, which are to beused as the source 120 and the drain 110, so that a source electrode anda drain electrode may be connected.

Referring to FIG. 5(d), the remaining area except for the areas to beused as the source 120 and the drain 110 is then removed from theP+-type diffusion layers 111 and 121 by etching. Electrodes are formedby depositing a metal or the like in the areas of the insulating oxidefilms 142 and 143 which have been removed in the source 120 and thedrain 110.

In the TFT photodetector 100 fabricated in the above manner, currentflows through a current channel excited between the source 120 and thedrain 110 by tunneling, as described before. If the thickness of theactive layer 130 is equal to or larger than a predetermined thickness,for example, 100 nm, a neutral area is produced separately in an areaunder the gate 150, which has not been depleted perfectly, except forthe current channel generated by light. Unnecessary extra chargesgenerated by light may be accumulated in the neutral area, and arelikely to act as a changing factor to the threshold voltage which islinearly changed by light. Therefore, the extra charges need to beprocessed separately.

FIG. 6 is an energy band diagram referred to for describing aphotoelectric conversion mechanism of a TFT photodetector according toan embodiment of the present disclosure.

When light is incident on the gate 150 as the light receiving part, EHPsare generated in the gate 150 and the active layer 130. Excitedelectrons of the active layer 130 tunnels through the insulating oxidefilm 140 by an electric field, thereby depleting a bottom end portion ofthe gate 150. As a result, the total charge amount of the gate 150 ischanged, which leads to a threshold voltage modulation effect equivalentto application of a negative power source to the gate 150. Accordingly,a current channel is formed in the active layer 130 of poly-Si, and thuscurrent flows between the source 120 and the drain 110. The TFTphotodetector 100 implemented based on this structure and principle hasa high-sensitivity detection capability of sensing even a single photonand enables very intense photocurrent to flow, even with a small amountof light.

FIG. 7 is an energy band diagram referred to for describing a tunnelingmechanism of a TFT photodetector according to an embodiment of thepresent disclosure.

In the TFT photodetector 100, the shielding layer 170 is formed suchthat only the active layer 130 facing the shielding layer 170 with thegate 150 serving as the light receiving part and the insulating oxidefilm 140 in between is affected by light, with no effect of light on theremaining area. The shielding layer 170 may be formed by a silicide andmetal process, and may not be formed on the gate 150 through a mask.

Light of multiple wavelengths incident on the TFT photodetector 100 ismostly transmitted through or absorbed to the gate 150 formed of poly-Sior a-Si.

If the thickness of the gate 150 is equal to or larger than apredetermined value, for example, 300 nm, short-wavelength light of theblue family in light incident on the TFT photodetector 100 is mostlyabsorbed to the gate 150, while only very partial short-wavelength lightreaches the active layer 130 under the gate 150.

As described above, since the TFT photodetector 100 has an excellenthigh-sensitivity detection capability compared to a conventionalphotodetector, even though only a very small part of light of a shortwavelength incident on the gate 150 is transmitted through the gate 150and reaches the active layer 130, the threshold voltage of the currentchannel is changed and thus even a slight change in light may be sensed.

Light of the other wavelengths is also transmitted through the gate 150and reaches the active layer 130 in the same principle. Accordingly, thesame phenomenon as observed from reception of light of a shortwavelength occurs to the gate 150, thereby causing a change in thethreshold voltage of the current channel. However, because light of arelatively long wavelength is easily transmitted through the gate 150and reaches the active layer 130, compared to light of a shortwavelength, the light of a long wavelength generates more EHPs in theactive layer 130. Therefore, more electrons migrate to the gate 150through the insulating oxide film 140 by tunneling, causing a change inthe threshold voltage of the current channel between the source 120 andthe drain 110.

The metal protection layer 160 formed between the transparent substrate342 and the active layer 130 blocks light introduced through thetransparent substrate 342 from reaching an area other than the activelayer 130. Therefore, the light is absorbed only to or transmitted onlythrough the active layer 130 contacting the gate 150, and thus efficienttunneling through the insulating oxide film 140 occurs.

For more efficient tunneling, a predetermined voltage may be appliedbetween the gate 150 of poly-Si and the active layer 130 of poly-Si, ora property such as dark current may be adjusted by adjusting a tunnelingprobability and controlling an initial threshold voltage of the TFTphotodetector 100.

Then, when the intensity of light is decreased or light is blocked,tunneled electrons are re-tunneled to the active layer 130, and thus theamount of charge in the gate 150 returns to an original level.Accordingly, the formed depletion layer is reduced and, at the sametime, photocurrent generated in the current channel is also reduced.

However, it may occur that charges have not completely disappeared andthus have remained in the active layer 130 even after the lightblocking, causing an error such as a signal delay in the next lightirradiation. To avert this problem, the thickness of the active layer130 may be controlled such that an area remaining as an intermediatearea, in which no channel is generated, may be reduced, or a resetdevice may be added to remove the charges remaining in the active layer130.

FIG. 8 illustrates a mechanism for photoelectric conversion caused by aplurality of localized states in a TFT photodetector formed of a-Si orpoly-Si.

FIG. 8(a) illustrates the energy band of general single crystallinesilicon, and FIG. 8(b) illustrates the energy bands of the gate and theactive layer of a TFT photodetector of a-Si or poly-Si.

In the TFT photodetector 100, electrons are introduced into the N-typegate 150 by tunneling from the P-type active layer 130 to the insulatingoxide film 140, among charges of the two PN areas excited with theinsulating oxide layer 140 in between, the electron migration changesthe threshold voltage of the current channel between the source 120 andthe drain 110 in correspondence with a change in the total amount ofcharge in the gate 150, the threshold voltage modulation effect causedby the change in the amount of charge in the light receiving part causesa change in the conductance of the current channel, and thusphotocurrent corresponding to the changed conductance flows.

As the gate 150 as the light receiving part and the active layer 130 areformed of a-SI or poly-Si, instead of single crystalline silicon,according to an embodiment of the present disclosure, a plurality oflocalized energy levels are formed in the gate 150 and the active layer130, thereby forming a wavelength extension layer 180 that extends thewavelength range of light absorbed by the TFT photodetector 100.

The wavelength extension layer 180 is formed of a-Si/poly-Si. Asillustrated in FIG. 8(b), a plurality of local energy levels aregenerated through multiple localized states formed in a forbidden bandbetween the conduction band and valence band of the gate 150 and theactive layer 130. The localized states are naturally generated in theforbidden band in view of the nature of the a-Si/poly-Si structure,which obviates the need for applying stress or implanting ion toartificially form the localized states. Therefore, processes aresimplified.

Accordingly, the TFT photodetector 100 may generate EHPs by absorbinglight even at an energy level lower than 1.12 eV which is the band gapenergy of the general single crystalline silicon, thereby enablingdetection of the wavelength range of the near-infrared area, which islonger than a maximum detectable wavelength of silicon, 1150 nm, anddetection of light of a wavelength that a general silicon photodiode isnot capable of detecting.

As described above, because the TFT photodetector 100 is formed of a-Sior poly-Si, compared to a conventional photodetector formed of singlecrystalline silicon, the wavelength extension layer 180 includingmultiple localized states in the forbidden band exists, and there is noneed for artificially forming localized states by applying uniaxialtensile stress on single crystalline silicon, combining hetero elements(e.g., Ge or the like), implanting ions (e.g., P, B, N, Ga, or thelike), or increasing the doping density of an oxide film, poly-Si,and/or a substrate to control a thermal process strength. Therefore, afabrication process is simplified.

As described before, the TFT photodetector 100 according to theembodiment of the present disclosure may generate a flow of photocurrentwith an intensity higher than the conventional photodetector by hundredsof times to a few thousands of times, for the same light intensity.

Further, because the TFT photodetector 100 according to the embodimentof the present disclosure includes a plurality of localized states, thewavelength range in which a valid signal is detectable is extended.Thus, the TFT photodetector 100 is applicable to a sensor for biometricrecognition, motion recognition, or the like.

While the TFT photodetector 100 has been described as implemented in asimilar structure to a P-channel metal-oxide semiconductor (PMOS), thisshould not be construed as limiting. The TFT photodetector 100 may beimplemented in a similar structure to an N-channel metal-oxidesemiconductor (NMOS) by exchanging the doping impurities of the gate andthe active layer.

With reference to FIGS. 9 to 14 , a unit pixel including a TFTphotodetector 100 will be described below in detail.

When a PAT-PD pixel is formed on a substrate by the above-described TFTprocess, various pixel structures may be used according to the thicknessof an active layer.

The substrate may be a glass substrate or a flexible substrate such as aPI film.

The active layer may contain a material with a conductive propertycontrollable by tunneling or an electric field. For example, the activelayer may contain at least one of a-Si or poly-Si.

According to the present disclosure, a transparent display panel capableof displaying and image sensing may be realized by vertically stacking ascreen panel of a display and an image sensing panel or arranging thescreen panel of the display and the image sensing panel on the samepanel.

Further, according to the present disclosure, a switching TFT fordisplay and a driving TFT for image sensing may be fabricated in asingle process by arranging the screen panel and the image sensing panelon the same panel.

Embodiments of the structure of a unit pixel according to the thicknessof an active layer will be described with reference to FIGS. 9 to 12 .

The pixel structure may vary based on a reference thickness value of theactive layer, for example, 100 nm.

FIGS. 9 and 10 are diagrams illustrating unit pixels, when the thicknessof an active layer is 100 nm or less.

A unit pixel 910 may include a TFT photodetector 911 including an activelayer formed of a-Si or poly-Si on an amorphous transparent substrate,and at least one transistor. In the embodiment of FIG. 9 , the unitpixel 910 may include transistors M1, M2 and M3 as the at least onetransistor.

When light is incident on the TFT photodetector 911, electrons may beintroduced into an N-type gate by tunneling from a P-type active layerto an oxide film, among charges of two PN areas excited with the oxidefilm in between. Further, the migration of the electrons changes thethreshold voltage of a current channel between a source and a drain incorrespondence with a change in the total amount of charge in the gatein the TFT photodetector 911. Further, photocurrent proportional to theintensity of the received light flows in the active layer according tothe changed threshold voltage in the TFT photodetector 911. The TFTphotodetector 911 may convert the flowing photocurrent to a voltageoutput. The transistors M1, M2, and M3 may be electrically coupled tothe TFT photodetector 911 and generate a voltage output from thephotocurrent generated from the active layer of the TFT photodetector911.

In FIG. 9 , the unit pixel 910 may convert photocurrent into a voltageoutput, using parasitic capacitance generated from the at least onetransistor.

Specifically, the unit pixel 910 may convert photocurrent into a voltageoutput, using parasitic capacitance generated between the transistors M1and M3.

The transistor M2 is a select transistor which may control charging of aparasitic capacitor.

Specifically, when the select transistor is turned on, the parasiticcapacitor may be charged with photocurrent resulting from photoelectricconversion of the TFT photodetector 911. The photocurrent charged in theparasitic capacitor may be realized as an image.

The select transistor M2 may reset signals, when RST is turned on in theturn-on state of the select transistor M2.

Specifically, when RST is turned on with the transistor M2 turned on,charges may be removed from total column buses and the TFT photodetector911 through a ground GND.

An integration time required for an actual image sensor may be definedby this operation, and continuous images may be obtained in a shutterscheme.

FIG. 10 illustrates a unit pixel 1010 that directly charges a capacitor,instead of parasitic capacitance.

Specifically, the unit pixel 1010 may directly charge a capacitor 1012coupled to a source follower with photocurrent generated from a TFTphotodetector 1011.

When light is incident on the TFT photodetector 1011, electrons may beintroduced into an N-type gate by tunneling from a P-type active layerto an oxide film, among charges of two PN areas excited with the oxidefilm in between. Further, the migration of the electrons changes thethreshold voltage of a current channel between a source and a drain incorrespondence with a change in the total amount of charge in the gatein the TFT photodetector 1011. Further, photocurrent proportional to theintensity of the received light flows in the active layer according tothe changed threshold voltage in the TFT photodetector 1011. The TFTphotodetector 1011 may convert the flowing photocurrent into a voltageoutput. Transistors M1, M2, and M3 may be electrically coupled to theTFT photodetector 1011 and generate a voltage output from thephotocurrent generated from the active layer of the TFT photodetector1011.

In the embodiment of FIG. 10 , larger capacitance than parasiticcapacitance may be used by means of a capacitor 1012. Further, as thelarge capacitance is controllable, the output characteristics of a widerdynamic range than in the embodiment of FIG. 9 may be achieved.

In the embodiment of FIG. 10 , at least one transistor may include aselect transistor M1.

When the select transistor M1 is turned on, the capacitor 1012 of an IVCcircuit among capacitors coupled to a transistor corresponding to asource follower may be charged.

Specifically, the capacitor 1012 of the IVC circuit within the pixel maybe charged with photocurrent resulting from photoelectric conversion ofthe TFT photodetector 1011 due to turn-on of the select transistor M1.

The photocurrent charged in the capacitor 1012 may be converted into avoltage and output as VOUT. The voltage VOUT may be provided in the formof a signal to a separate driving circuit such as a co-double sampling(CDS) circuit.

The signal may be reset by the select transistor M1.

For example, when RST (M2) is turned on with the select transistor M1turned on, charges may be removed from the capacitor 1012 of the IVCcircuit, total column buses, and the TFT photodetector 1011 through aground GND.

An integration time required for an actual image sensor may be definedby this operation, and continuous images may be obtained in a shutterscheme.

For example, the TFT photodetector 1011 used in FIG. 10 may be formed ina poly-Si active layer of a thickness less than 100 nm on a glasssubstrate. Therefore, a perfectly depleted current channel area may berealized.

Further, the formation of the perfectly depleted current channel area inthe TFT photodetector 1011 obviates the need for using a separatedetector transistor for resetting.

FIGS. 11 and 12 are diagrams illustrating unit pixels, when thethickness of an active layer is equal to or larger than 100 nm.

When a poly-Si active layer is fabricated to a thickness equal to orlarger than 100 nm on a glass substrate in a fabrication process, aneutral area is separately formed in a lower part of a gate, which isnot completely depleted, aside from a current channel generated bylight.

Unnecessary extra charges generated by light may be accumulated in thisneutral area. The accumulated charges are likely to act as a factor thatchanges a threshold voltage which changes linearly by light.

The residual charges may be controlled by directly coupling a separatetransistor to the active layer.

For this purpose, referring to FIG. 11 , a unit pixel 1110 includes aTFT photodetector 1111 with a poly-Si active layer formed to a thicknessequal to or larger than 100 nm.

When light is incident on the TFT photodetector 1111, electrons may beintroduced into an N-type gate by tunneling from a P-type active layerto an oxide film, among charges of two PN areas excited with the oxidefilm in between. Further, the migration of the electrons changes thethreshold voltage of a current channel between a source and a drain incorrespondence with a change in the total amount of charge in the gatein the TFT photodetector 1111. Further, photocurrent proportional to theintensity of the received light flows in the active layer according tothe changed threshold voltage in the TFT photodetector 1111. The TFTphotodetector 1111 may convert the flowing photocurrent into a voltageoutput. Transistors M1 and M2 may be electrically coupled to the TFTphotodetector 1111 and generate a voltage output from the photocurrentgenerated from the active layer of the TFT photodetector 1111.

A transistor M3 is directly coupled to the active layer of the TFTphotodetector 1111.

The transistor M3 may be configured to include a gate end connected toVDD, a drain end connected to the active layer of the TFT photodetector1111, and a source end connected to SCG. That is, as VDD is input to thegate end, unnecessary extra residual charges accumulated in the poly-Siactive layer of the TFT photodetector 1111 may flow from the drain endto the source end and may be controlled through an SCG channel.

Referring to FIG. 12 , a unit pixel 1210 includes a TFT photodetector1211 with a poly-Si active layer formed to a thickness equal to orlarger than 100 nm.

When light is incident on the TFT photodetector 1211, electrons may beintroduced into an N-type gate by tunneling from a P-type active layerto an oxide film, among charges of two PN areas excited with the oxidefilm in between. Further, the migration of the electrons changes thethreshold voltage of a current channel between a source and a drain incorrespondence with a change in the total amount of charge in the gatein the TFT photodetector 1211. Further, photocurrent proportional to theintensity of the received light flows in the active layer according tothe changed threshold voltage in the TFT photodetector 1211. The TFTphotodetector 1211 may convert the flowing photocurrent into a voltageoutput. Transistors M1, M2 and M3 may be electrically coupled to the TFTphotodetector 1211 and generate a voltage output from the photocurrentgenerated from the active layer of the TFT photodetector 1211.

A transistor M4 is directly coupled to the active layer of the TFTphotodetector 1211.

The transistor M4 may be configured to include a gate end connected toRST, a drain end connected to the active layer of the TFT photodetector1211, and a source end connected to RST. That is, as RST is input to thegate end, unnecessary extra residual charges accumulated in the poly-Siactive layer of the TFT photodetector 1211 may flow from the drain endto the source end and thus may be reset.

FIG. 12 illustrates a unit pixel 1210 that directly charges a capacitor1212, instead of parasitic capacitance.

Specifically, the unit pixel 1210 may directly charge the capacitor 1212coupled to a source follower with photocurrent generated from the TFTphotodetector 1211.

In the embodiment of FIG. 12 , larger capacitance than parasiticcapacitance may be used by means of the capacitor 1212. Further, as thelarge capacitance is controllable, the output characteristics of a widerdynamic range may be achieved.

The capacitor 1212 of the IVC circuit within the pixel may be chargedwith photocurrent resulting from photoelectric conversion of the TFTphotodetector 1211 due to turn-on of the select transistor M1.

The photocurrent charged in the capacitor 1212 may be converted into avoltage and output as VOUT. The voltage VOUT may be provided in the formof a signal to a separate driving circuit such as a CDS circuit.

The signal may be reset by the select transistor M1.

For example, when RST (M2) is turned on with the select transistor M1turned on, charges may be removed from the capacitor 1212 of IVC, totalcolumn buses, and the TFT photodetector 1211 through a ground GND.

When the unit pixel 1110 or 1210 is used, a light source for the displaymay also be used as a light source for an image sensor. Further, the BLUof an LCD or a light emitting source of an OLED may also be used.Further, it is possible to detect light in the wavelength range of thenear-infrared area, which is longer than a maximum detectable wavelengthof general silicon, 1150 nm.

FIG. 13 is a diagram illustrating an active pixel system (APS)-type cellarray in which unit pixels are arranged to output a voltage to a drivingcircuit.

A display panel may be formed by arranging multiple unit pixels asillustrated in FIGS. 9 to 12 in an array. An image signal may begenerated by the thus-formed screen panel.

Referring to FIG. 13 , a two-dimensional image may be obtained from unitpixels according to an embodiment of the present disclosure. The unitpixels may be arranged in an array, as illustrated in FIG. 13 . Further,an image signal may be obtained by a horizontal scanner 1310 fordetecting a signal of each column, a vertical scanner 1320 for detectinga signal of each row, and an analog signal driving circuit including ananalog-to-digital converter (ADC), which is coupled to each scanner.Particularly, a scheme of providing a voltage output of each unit pixelto the driving circuit corresponds to an APS.

The APS is robust against external noise.

The driving circuit may be installed outside the screen panel, apartfrom an array of unit pixels, and may further include circuits such as aBLC, CDS, PGA, S&H, PLL, and ADC.

FIG. 14 is a diagram illustrating a passive pixel system (PPS)-type cellarray in which unit pixels are arranged to provide a current output to adriving circuit according to an embodiment of the present disclosure.

Referring to FIG. 14 , a two-dimensional image may be obtained from unitpixels according to an embodiment of the present disclosure. The unitpixels may be arranged in an array, as illustrated in FIG. 14 . Further,an image signal may be obtained by a horizontal scanner 1410 fordetecting a signal of each column, a vertical scanner 1420 for detectinga signal of each row, and an analog signal driving circuit including anADC, which is coupled to each scanner. Particularly, a scheme ofproviding a voltage output of each unit pixel to the driving circuitcorresponds to a PPS.

The PPS is a structure capable of a high-sensitivity image in a widedynamic range.

The driving circuit may include circuits such as an IVC, BLC, CDS, PGA,S&H, PLL, and ADC.

As is apparent from the foregoing description, according to variousembodiments of the present disclosure, a high-sensitivity image sensormay be realized on a glass substrate or a flexible substrate such as aPI film, which is used as a display panel, by a TFT fabricationtechnology.

According to an embodiment, the display module may function as an imagesensor, which obviates the need for providing a separate image sensor ona display panel.

According to an embodiment, a switching TFT for display and a drivingTFT for image sensing may be fabricated in a single process by arranginga screen panel and an image sensing panel on the same panel.

According to an embodiment, a light source for a display may also beused as a light source for an image sensor.

According to an embodiment, as both of a BLU of an LCD and a lightemitting source of an OLED may be used, image sensing may be processedwithout the need for providing a separate light emitting part requiredfor an image sensor.

According to an embodiment, it is possible to detect light in thewavelength range of the near-infrared area, which is longer than amaximum detectable wavelength of general silicon, 1150 nm.

While the disclosure has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the disclosure asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A unit pixel arranged along with a display pixelin each pixel of a display panel, the unit pixel comprising: a thin-filmtransistor (TFT) photodetector including an active layer formed ofamorphous silicon or polycrystalline silicon on an amorphous transparentsubstrate; and at least one transistor electrically coupled to the TFTphotodetector and configured to generate a voltage output fromphotocurrent generated from the active layer, wherein the at least onetransistor converts the photocurrent into the voltage output based onparasitic capacitance between the at least one transistor, caused by thephotocurrent generated from the active layer.
 2. The unit pixel of claim1, wherein the TFT photodetector is configured in a structure in whichwhen light is incident, electrons are introduced into an N-type gate bytunneling from a P-type active layer to an insulating oxide film, amongcharges of two PN areas excited with the insulating oxide film inbetween, the electron migration changes a threshold voltage of a currentchannel between a source and a drain in correspondence with a change inthe total amount of charge in the gate, the photocurrent proportional tothe intensity of the received light flows in the active layer, and thevoltage output is generated from the flowing photocurrent.
 3. The unitpixel of claim 1, wherein the active layer contains a material with aconductive property controllable by tunneling or an electric field. 4.The unit pixel of claim 3, wherein the active layer contains at leastone of amorphous silicon or polycrystalline silicon.
 5. The unit pixelof claim 1, wherein the at least one transistor includes a selecttransistor, and wherein when the select transistor is turned on, theparasitic capacitance is generated.
 6. A unit pixel, arranged along witha display pixel in each pixel of a display panel, the unit pixelcomprising: a thin-film transistor (TFT) photodetector including anactive layer formed of amorphous silicon or polycrystalline silicon onan amorphous transparent substrate; and at least one transistorelectrically coupled to the TFT photodetector and configured to generatea voltage output from photocurrent generated from the active layer,wherein the at least one transistor converts photocurrent into thevoltage output, the photocurrent being generated by directly charging acapacitor coupled to a transistor corresponding to a source followerwith the photocurrent generated from the active layer.
 7. The unit pixelof claim 6, wherein the at least one transistor includes a selecttransistor, and wherein when the select transistor is turned on, acapacitor of an IVC circuit is charged, among capacitors coupled to thetransistor corresponding to the source follower, and the chargedphotocurrent is converted into a voltage which is an output of the IVCcircuit.
 8. The unit pixel of claim 1, wherein a pixel structure isdetermined based on a thickness of the active layer.
 9. The unit pixelof claim 8, further comprising a transistor coupled to the active layerand configured to control residual charges in a neutral area, when thethickness of the active layer is equal to or larger than a referencevalue.
 10. The unit pixel of claim 9, wherein the reference value is 100nm.
 11. The unit pixel of claim 1, wherein the TFT photodetectorcomprises: an amorphous transparent substrate used as a display panel; asource formed of amorphous silicon or polycrystalline silicon on thetransparent substrate; a drain formed of amorphous silicon orpolycrystalline silicon, opposite to the source on the transparentsubstrate; an active layer formed between the source and the drain andhaving a current 30 channel formed between the source and the drain; aninsulating oxide film formed on the source, the drain, and the activelayer; and a light receiving part formed on the insulating oxide film,isolated from the active layer by the insulating oxide film, andconfigured to absorb light.
 12. The unit pixel of claim 11, wherein inthe TFT photodetector, when light is incident on the light receivingpart, electrons migrate by tunneling through the insulating oxide filmbetween the light receiving part and the active layer which have beenexcited with the insulating oxide film in between, the amount of chargein the light receiving part is changed by the migration of theelectrons, a threshold voltage of the current channel is changed due tothe change of the amount of charge, and photocurrent flows through thecurrent channel due to the change of the threshold voltage.
 13. The unitpixel of claim 1, wherein the TFT photodetector uses light generatedfrom the display panel as a light source for a sensor.
 14. A unit pixelof a display panel, comprising: a display sub-panel including a lightemitting device; and an image sensor sub-panel stacked with the displaysub-panel, and configured to generate a voltage output from photocurrentgenerated from an active layer in response to sensed light, wherein theimage sensor sub-panel comprises: a thin-film transistor (TFT)photodetector including an active layer formed of amorphous silicon orpolycrystalline silicon on an amorphous transparent substrate; and atleast one transistor electrically coupled to a source side of the TFTphotodetector and configured to generate a voltage output fromphotocurrent generated from the active layer, wherein the at least onetransistor converts the photocurrent into the voltage output based onparasitic capacitance between the at least one transistor, caused by thephotocurrent generated from the active layer.
 15. The unit pixel ofclaim 14, wherein the image sensor sub-panel is configured in astructure in which when light is incident, electrons are introduced intoan N-type gate by tunneling from a P-type active layer to an insulatingoxide film, among charges of two PN areas excited with the insulatingoxide film in between, the electron migration changes a thresholdvoltage of a current channel between a source and a drain incorrespondence with a change in the total amount of charge in the gate,the photocurrent proportional to the intensity of the received lightflows in the active layer, and the voltage output is generated from theflowing photocurrent.